1. Technical Field
The present invention relates to a light emitting device and a display unit.
2. Description of the Background
The behavior of nanomaterials is intermediate between those of an atom or a molecular and those of a macroscopic solid (bulk form). Nanomaterials having charge carriers and excitation confined in all three dimensional directions are referred to as quantum dots. Since a quantum dot has increased effective band gaps as its size decreases, a spectrum having its absorption wavelength and emission wavelength shifted to the short wavelength side can be obtained. In addition, by controlling both the composition of a quantum dot in addition to its size, a spectrum having a wide range of from infrared to ultraviolet can be obtained. Moreover, by controlling the size distribution, a spectrum having excellent color purity with a narrow half value width can be obtained.
For these reasons, it is expected that the quantum dot can find applications in labeling and imaging of biological molecules and light emitting devices such as a photoluminescent devices and electroluminescent device. However, insufficient luminance efficiency is a problem.
Therefore, Japanese patent application publication no. 2009-99545 (JP-2009-99545-A) describes an electroluminescence device having a pair of electrodes and an electroluminescent layer containing a luminous layer arranged between the pair of electrodes. The luminous layer has a structure in which quantum dots having a surface protected by a protective material are dispersed in a matrix material containing an organic compound. In addition, a first protective material is contained as the protective material that satisfies the following relationships (A), (B), and (C):Ip(h)<Ip(m)+0.1 eV  Relationship (A)Ea(h)>Ea(m)−0.1 eV  Relationship (B)Eg(q)<Eg(h)<Eg(m)  Relationship (C)
In the relationships, Ip(h) represents the absolute value of the ionization potential of the first protective material, Ip(m) represents the absolute value of the ionization potential of the organic compound contained in the matrix material, Ea(h) represents the absolute value of the electronic affinity of the first protective material, Ea(m) represents the absolute value of the electronic affinity of the organic compound, Eg(h) represents the bandgap of the first protective material, Eg(m) represents the bandgap of the organic compound, and Eg(q) represents the bandgap of the quantum dot.
Furthermore, when the first protective material has at least one remaining hydrophilic group and a hydrophobic group in one molecule, specific examples of the combination of the hydrophobic group of the protective material/organic compound/quantum dot include:
(1) remaining group of 3-(2-benzo thiazoleyl)-7-(diethylamino) coumalin/4,4′-bis(carbazole-9-yl)biphenyl/CdSe/ZnS (luminous wavelength: 620 nm);
(2) remaining group of 5,6,11,12-tetraphenyl naphthacene/4,4′-bis(carbazole-9-yl)biphenyl/CdSe/Zns (luminous wavelength: 620 nm); and
(3) remaining group of 2,5,8,11-tetra-tert-butyl perylene/4,4′-bis(carbazole-9-yl-biphenyl/CdSe/ZnS (luminous wavelength: 520 nm).
However, even with these structures the luminance efficiency remains insufficient.